Scholars have consistently demonstrated that the socioeconomic composition of the pupil body is related to academic achievement. The effect of ethnic/immigrant concentration, on the other hand, is more controversial, as some have found no impact of the ethnic/immigrant composition when other aspects were taken into account. Social capital theory claims that it is possible to compensate for a disadvantaged background or school composition when pupils benefit from being integrated in specific social structures. This article tests whether social capital is positively related to the mathematics achievement of pupils in the fourth and fifth grades of Flemish primary schools in which most of the pupils have a low socioeconomic and/or an ethnic/immigrant background (i.e. segregated schools).